The invention relates to wireless communications systems. In particular, the invention relates to reducing the power consumption in a remote unit in a wireless communication system utilizing slotted paging.
A wireless communication system may comprise multiple remote units and multiple base stations. FIG. 1 exemplifies an embodiment of a terrestrial wireless communication system with three remote units 10A, 10B and 10C and two base stations 12. In FIG. 1, the three remote units are shown as a mobile telephone unit installed in a car 10A, a portable computer remote 10B, and a fixed location unit 10C such as might be found in a wireless local loop or meter reading system. Remote units may be any type of communication unit such as, for example, hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link 14 from the base station 12 to the remote units 10 and a reverse link 16 from the remote units 10 to the base stations 12.
Communication between remote units and base stations, over the wireless channel, can be accomplished using one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). An industry standard for CDMA is set forth in the TIA/EIA Interim Standard entitled xe2x80x9cRemote unit-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, TIA/EIAJIS-95, and its progeny (collectively referred to here as IS-95), the contents of which are incorporated by reference herein in their entirety. Additional information concerning a CDMA communication system is disclosed in U.S. Pat. No. 4,901,307, entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS, (the ""307 patent) assigned to the assignee of the present invention and incorporated in its entirety herein by reference.
In the ""307 patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through base stations using CDMA spread spectrum communication signals. The CDMA modulation techniques disclosed in the ""307 patent offer many advantages over other modulation techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In a typical CDMA communication system, remote units only sporadically establish bi-directional communication with a base station. For example, a cellular telephone remains idle for significant periods of time when no call is in process. To ensure that any message directed to a remote unit is received, the remote unit must continuously monitor the communication channel even while it is idle. For example, while idle, the remote unit monitors the forward link channel from the base station to detect incoming calls. During such idle periods, the cellular telephone continues to consume power to sustain the elements necessary to monitor for signals from the base stations. Many remote units are portable and are powered by an internal battery. For example, personal communication system (PCS) handsets are almost exclusively battery-powered. The consumption of battery resources by the remote unit in idle mode decreases the battery resources available to the remote unit when a call is placed or received. Therefore, it is desirable to minimize power consumption in a remote unit in the idle state and thereby increase battery life.
One means of reducing remote unit power consumption in a communication system is disclosed in U.S. Pat. No. 5,392,287, entitled APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE COMMUNICATION RECEIVER (the ""287 patent), assigned to the assignee of the present invention and hereby incorporated in its entirety herein by reference. In the ""287 patent, a technique for reducing power consumption in a remote unit operating in an idle mode (i.e. a remote unit which is not engaged in bi-directional communication with a base station) is disclosed. In idle, each remote unit periodically enters an xe2x80x9cactivexe2x80x9dstate during which it prepares to and receives messages on a forward link communication channel. In the time period between successive active states, the remote unit enters an xe2x80x9cinactivexe2x80x9d state. During the remote unit""s inactive state, the base station does not send any messages to that remote unit, although it may send messages to other remote units in the system that are in the active state.
As disclosed in the ""287 patent, a base station broadcast messages which are received by all remote units within the base station coverage area on a xe2x80x9cpaging channel.xe2x80x9d All idle remote units within the base station coverage area monitor the paging channel. The paging channel is divided in the time dimension into a continuous stream of xe2x80x9cslots.xe2x80x9d Each remote unit operating in slotted mode monitors only specific slots which have been assigned to it as active (assigned) slots. The paging channel continually transmits convolutional encoded messages in numbered slots, repeating the slot sequence, such as for example, every 640 slots. When a remote unit enters the coverage area of a base station, or if a remote unit is initially powered on, it communicates its presence to a preferred base station. Typically the preferred base station is the base station which has the strongest pilot signal as measured by the remote unit.
The preferred base station, along with a plurality of geographically near neighboring base stations, assign a slot, or a plurality of slots, within their respective paging channels, for the remote unit to monitor. The base station uses the slots in the paging channel to transmit control information to a remote unit, if necessary. The remote unit may also monitor a timing signal from the preferred base station allowing the remote unit to align, in the time dimension, to the base station slot timing. By aligning in the time dimension to the preferred base station slot timing, the remote unit can determine when a paging channel slot sequence begins. Thus, knowing when the paging channel slot sequence begins, which slots are assigned for it to monitor, the total number of slots in the repetitive paging channel sequence of slots, and the period of each slot, the remote unit is able to determine when its assigned slots occur.
Generally, the remote unit is in the inactive state while the base station is transmitting on the paging channel in slots which are not within the remote unit""s assigned set. While in the inactive state, the remote unit does not monitor timing signals transmitted by the base station, maintaining slot timing using an internal clock source. Additionally, while in the inactive state the remote unit may remove power and/or clocks from selected circuitry, such as, for example, circuits which monitor the wireless channel and the decoder. Using its internal timing, the remote unit transits to its active state a short period of time before the next occurrence of an assigned slot.
In order for a message to be decoded with high reliability by the remote unit, the active state must include the time needed to reinitialize the receiver chain to provide valid receive samples, the time needed to search these samples for multipath and assign demodulation fingers to provide a valid symbol stream, and the time needed to initialize the state metrics with the symbol stream before the symbols associated with the message of interest. During the initialization of the analog receiver chain, the frequency synthesizer used to mix the received signal down to baseband frequency must come into lock, and gain scaling and DC bias loops, if any, must lock to provide a valid baseband receive sample stream. In a communication system based on IS-95, the paging channel is continuously encoded with a K=9, rate {fraction (1/2)} convolutional code. Several constraint lengths of symbols must be provided to the Viterbi decoder to initialize its state metric values in advance of the symbols of interest.
To demodulate the paging channel, the remote unit needs to acquire a precise timing reference for each of the multipath instances that comprise the received waveform. Demodulator fingers, each with their own specific time reference, are assigned to the individual multipath instances. Each, finger despreads, the receive samples at its assigned time reference. The despread results from each finger are then combined to form a single symbol stream for decoding. Such a demodulator, known as a RAKE receiver, is described in U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, issued Apr. 28, 1992, assigned to the assignee of the present invention and incorporated herein by reference.
During initial power-on acquisition, all possible shifts of the PN sequences are searched to acquire the timing references for each finger. Searching each PN offset can take anywhere from hundreds of milliseconds to a few seconds depending on the channel conditions during acquisition. Performing such a full reacquisition at the start of every active state would take too long and consume too much current for a practically sized portable battery. Instead, the clock to the demodulator circuit is gated off for a precise duration of time which is designed so that the circuits are automatically aligned with the system when their clocks are gated back on. The time period is measured by a sleep timer clocked off a high precision oscillator. During the sleep interval, only the oscillator and the sleep timer are active.
In IS-95 systems, an interleaver in the transmitter and a deinterleaver in the remote unit process data on 20 ms frame boundaries. Both must be aligned with each other. PN generators in the transmitter and the remote unit have sequence lengths of 215. Each PN generator updates at a rate of 1.2288 MHz, therefore the PN sequence period is 26.66 ms. PN generators in the remote unit must align with respective PN generators in the transmitter. The smallest period that is common to both the interleaver/deinterleaver timing and the PN sequence timing is 80 ms. Exactly 3 PN sequence periods of 26.66 ms and exactly 4 interleaver frames of 20 ms fit into an 80 ms period. More generally, the sleep interval is programmed in steps of the least common multiple of the two intervals.
The sleep timer in the remote unit is programmed to sleep for a multiple of the 80 ms period to ensure that both the timing reference of the fingers and the frame reference timing of the deinterleaver do not change with respect to actual time, or xe2x80x9cwall clockxe2x80x9d time, i.e., the timing of the preferred base station. If the sleep timer is programmed with a value other than a multiple of 80 ms, when the demodulator activates the PN generators and/or the interleaver timing will not be aligned with the proper system time and demodulation will be impossible.
To prolong battery life, it would be desirable to decrease the amount of time the remote unit is in its active state. However, the remote unit must be in its active state long enough, prior to the beginning of a message directed to it, to ensure that both the timing reference of the demodulation fingers and the frame reference timing of the deinterleaver are properly aligned. Therefore, there is a need in the art for a method and apparatus to decrease the time required by a remote unit to prepare for the receipt of messages during its assigned time slot.
The invention addresses these and other needs by providing a system and method wherein a method and apparatus which provide synchronization after a jump to a new time alignment. This allows, for example, a remote unit to enter an active state at a later time while still maintaining an accurate time sense.
In one aspect of the invention, a method is provided for use in a remote unit which operates in a slotted mode communication system. The method includes entering an active state from an inactive state after a predetermined time period. The combiner time counter and at least one demodulation finger are reactivated. An 80 ms counter is adjusted to a desired count. The combiner time counter is adjusted to the proper phase. Wall clock time is determined with reference to a combiner offset value, the 80 ms counter, and the combiner time counter.
In another aspect of the invention, a remote unit is provided which can provide synchronization after a jump to a new time alignment. The remote unit includes a demodulation finger, a combiner time counter, an 80 ms counter and a controller. The controller is configured to update the 80 ms to a desired count upon entering an active state after a predetermined period in an inactive state. The controller is further configured to force the combiner counter to the proper phase upon entering the active state. The combiner is also configured to calculate a time measurement with reference to the 80 ms counter in the correction factor.